1. Field of the Invention
This invention concerns a crucible made of carbon fiber-reinforced carbon composite material for use in pulling silicon single crystals apparatus by a Czochralski method (hereinafter referred to as a CZ method).
2. Related Art
In the CZ method, silicon single crystals are formed by pulling up molten silicon in crucibles. For the crucibles, those made of quartz having characteristics of high melting point, high temperature stability and less reactivity with molten silicon have been used. FIG. 6 shows a schematic view of a CZ silicon single crystal pulling apparatus. The outline for the CZ method is to be explained with reference to the drawing.
In FIG. 6, a quartz crucible 42 filled with polycrystalline silicon is placed at the inside of a crucible 43 in a chamber 41. The crucible 43 is placed on a susceptor 46 and pivoted on a pedestal 47, and situated centrally of the chamber 41. Further, a heater 44 and a temperature keeping cylinder 45 are disposed so as to surround the outer periphery of the crucible 43 to melt the polycrystalline silicon in the quartz crucible 42 by heating. The temperature for the polycrystalline silicon is directly measured by a pyrometer 48, by which temperature is controlled. Then, a seed crystal 50 attached to a seed chuck 49 is immersed in the molten polycrystalline silicon in the quartz crucible 42, and pulled up under rotation in the direction identical with or opposite to that of the crucible 43 to grow silicon single crystals.
For the production of silicon single crystals by the CZ method, a quartz crucible for melting silicon in the inside and a crucible made of graphite for containing and supporting the quartz crucible from the outside are used. In recent years, since the diameter of the single crystals to be manufactured is increased, the size of the pulling apparatus used in the CZ method is also increased. Correspondingly, the size of the graphite crucible is enlarged and the weight thereof is increased making it difficult for handling. Further, it has also resulted in a problem that the effective processing size in the pulling apparatus is decreased.
Carbon fiber-reinforced carbon composite material (hereinafter referred to as a C/C composite material) is light in weight compared with the graphite material and has excellent characteristics in various kinds of mechanical strengths. Accordingly, the processing chamber of the apparatus can be used effectively. Further, owing to the reduced weight, handlability such as installation to the apparatus is also excellent. In view of the above, the material of furnace components including the crucible used for the single crystal pulling apparatus of a large diameter by the CZ method have now been shifted from graphite to the C/C composite material The method of manufacturing a crucible made of the C/C composite material mainly includes the following two methods.
One of them is a method referred to as a filament winding method. In this method, after immersing a carbon fiber bundle formed by bundling carbon fibers into a binder of low viscosity comprising, for example, a thermosetting resin and a solvent and then winding the carbon fiber bundle deposited with the binder around a mandrel having the shape conforming a crucible thereby forming the same into a required crucible shape. Then, heat setting is conducted, for example, at a temperature of about 100 to 300xc2x0 C. and the resultant shaped product is carbonized in an inert gas such as N2 gas at a temperature, for example, of about 1000xc2x0 C. After the carbonization, a phenol resin, tar pitch or the like is impregnated optionally and then heating is applied at a temperature of 1500xc2x0 C. or higher to conduct carbonization (graphitization). The crucible obtained by steps described above is heated, for example, in a halogen gas atmosphere at a temperature of about 1500 to 2500xc2x0 C. to apply a high purity treatment and obtain a crucible comprising a C/C composite material.
The other is method referred to as a hand layup method. In this method, carbon fiber cloths are appended to the mold of a crucible to manufacture a molded product and then applied with thermosetting, carbonization, graphitization and high pure treatments to obtain a crucible made of a C/C composite material.
Generally, the C/C composite material is manufactured by combining the filament winding method and the hand layup method.
By the way, when the crucible 43 is actually used, it is necessary that the bottom thereof is flattened such that it can be placed stably on the susceptor 46 (refer to FIG. 6). For this purpose, as shown in FIG. 7, the bottom 52 of a crucible 51 has been flattened by cutting off a hatched portion 53 in FIG. 7 by machining. In this case, carbon fibers at the crucible bottom 52 are cut. Accordingly, this causes deformation of the crucible bottom 52 by the plastic deformation or the like of the C/C composite material per se or detachment of the carbon fibers. Then, when the crucible bottom 52 is deformed, gaps are formed to the C/C composite material crucible 51 themselves and between the C/C composite material crucible 51 and the susceptor 46 (refer to FIG. 6). SiO gases intrude into the gaps upon pulling operation and react with the crucible to form SiC, thereby resulting in a problem of consuming the crucible. Further, the carbon fibers at the cut portion 53 occasionally suffer from defoliation by the stresses upon deformation.
This invention has been accomplished for overcoming the foregoing problems and intends to provide a crucible made of a C/C composite material for use in single crystal pulling apparatus that can decrease the amount of machining as less as possible for the bottom of the crucible after forming the crucible by the filament winding method or the like used for single crystal pulling apparatus and suppress detachment or deformation of carbon fibers during use.
For overcoming the foregoing problems, this invention provides a crucible made of a C/C composite material for use in single crystal pulling, the crucible having a lateral cylindrical portion and a bottom portion integrally formed as multiple layers wound by a filament winding method, in which the first layer as the innermost crucible layer, among the multiple layers, is wound such that carbon fibers form tracks passing the polar point at the bottom, the second layer wound on the outer surface of the first layer is wound along a track to form a first outer circular bottom that extends outwardly from about a middle part of a raised portion where the carbon fibers of the first layer are localized to the polar point, and the third and the succeeding layers wound on the outer surface of the second layer are wound respectively along tracks to form outer circular bottoms that extend stepwise outwardly from about the middle parts of the outer surfaces of layers situated inside the respective layers, and the top for the raised portion of the first layer and the top for each of the outer circular bottoms at the bottom of the carbon fibers wound around as the multiple layers are at an substantially identical height. Further, the top may be flattened by machining.
According to the crucible made of a C/C composite material of this invention, the bottom of the crucible forms a bottom of a substantially identical height after molding by a filament winding method, which can be placed stably on a susceptor. Further, a flat surface can be formed by machining a portion of the tops for the protrusions on a corrugating surface formed slightly at the bottom to a required minimum extent. Further, when the amount of cutting by machining is restricted to 40% or less, preferably, 30% or less for the thickness of the bottom (thickness for the top of the raised portion of the first layer), the mechanical strength at the bottom is not lowered, detachment or defoliation of the carbon fibers during use is decreased and the deformation of the crucible per se made of the C/C composite material can also be suppressed. This can suppress conversion of the crucible into SiC upon pulling single crystals.
Further, this invention also provides a crucible made of a C/C composite material for single crystal pulling use, the crucible having a lateral cylindrical portion and a bottom portion formed integrally as multiple layers wound by a filament winding method, in which a substantially disk-shaped or a circular cylindrical bottom central member comprising a carbonaceous material is disposed at a central portion of the bottom, the first layer of the carbon fibers as the innermost crucible layer or plural layers including the second layer and the succeeding layers, among the multiple layers, are wound so as to cover the lateral periphery of the bottom central member along the circumferential surface thereof up to the height substantially identical with the bottom of central member, the respective layers after the layer of the carbon fibers wound up to the height substantially identical with the bottom of the bottom central member are would along tracks to form outer circular bottoms that extend stepwise outwardly from the layers situated inside of the respective layers, and the tops for the central bottom member and for each of the outer circular bottoms at the bottom of the carbon fibers wound around as the multiple layers are at an substantially identical height.
As described above, since the bottom central member comprising the carbonaceous material is disposed at the center of the bottom, the stability of the bottom can be improved further. Further, an opening can be formed to the bottom central portion without cutting the crucible-forming carbon fibers.